Generally, a tubular biaxial stretched film is produced by first passing a tubular non-stretched film through two pairs of nip rolls. The tubular non-stretched film is then heated to a predetermined temperature with a heater attached in a zone between the two pairs of nip rolls. Pressurized air is admitted to form a bubble. The film is then drawn so as to pass through a group of collapser rolls with a pair of lower nip rolls.
The pressurized air is, in general, admitted only at the start. After a bubble having a predetermined size is formed, a fixed quantity of pressurized air is kept in the bubble by shutting the lower nip rolls, and the stretching is continued.
However, in the above-mentioned method for producing biaxial stretched film where the air is trapped to keep the bubble, the bubble gradually becomes deflated since during the stretching process, the pressurized air (small quantity) leaks out through the upper or lower nip roll portions. Therefore, an opening angle of the group of collapser nip rolls is adjusted so that the lateral magnification is suitably kept without dropping. However, in some cases, even though the opening angle of the collapser nip rolls is adjusted, the adjustment provides merely a change of stretching point and does not contribute to increase the transverse magnification. Further, when the start-operation cannot provide the predetermined size of the bubble, it is necessary to repeat the start-operation again, and the repetition is problematic. Particularly, in a case where gas permeable film is used, though the upper and lower nip rolls are shut so as to exclose the pressurized air in the bubble, the interior air leaks through the film face. Then, the bubble adjusted to the predetermined size becomes gradually deflated, and the biaxial stretching process cannot be continued.
As a measure against those problems, there have been conventionally proposed methods where pressurized air is continuously admitted in the bubble, for example, as disclosed in Japanese Examined Patent Publication No. 3727/1958, Japanese Unexamined Patent Publication No. 139965/1979, and the like.
For example, Japanese Examined Patent Publication No. 3727/1958 etc. discloses an apparatus having lower nip rolls provided with a continuous annular groove and a method to supply pressurized air through a tubular insert member by inserting the insert member into the groove. However, the film is easily deformed with frictional force when the tubular insert member comes in contact with the film. Moreover, a portion of the film which is saved from contact and is pinched and drawn by the nip rolls receives transverse stretching according to the diameter of the tubular insert member. Therefore, uniformity of the film decreases, the flatness of the film descends, and the like.
On the other hand, the above-mentioned Japanese Unexamined Patent Publication No. 139965/1979 discloses a method where the film is cut at a position just above the lower nip rolls and pressurized air is admitted through an air injecting nozzle inserted into the cut portion. However, the method's disadvantage is that puncture of the bubble easily happens, since the film is cut after the bubble is formed. Particularly, in such a film that resistance to tearing is poor, it is impossible to continue the stretching since the torn portion will extend widely.